Pelizaeus-Merzbacher disease (PMD) is a leukodystrophy linked to the proteolipid protein gene (PLP). We report a cellular basis for the distinction between two disease subtypes, classical and connatal, based on protein trafficking of the two PLP gene products (PLP and DM20). Classical PMD mutations correlate with accumulation of PLP in the ER of transfected COS-7 cells while the cognate DM20 traverses the secretory pathway to the cell surface. On the other hand, connatal PMD mutations lead to the accumulation of both mutant PLP and DM20 proteins in the ER of COS-7 cells with little of either isoform transported to the cell surface. Moreover, we show that transport-competent mutant DM20s facilitate trafficking of cognate PLPs and hence may influence disease severity.